There is now a source code view. It is accessible via the "HTML Source" command under the "Edit" menu. You can drag-drop WYSIWYG page items into this view and they will automatically be converted to source code, and vice versa.
Note that it shows you PageMill's interpretation of the source code, not
what is actually on your disk until you save. For example, if you open a
file with <CENTER>, PageMill will display it as in the source view
as <P ALIGN=CENTER> even if you haven't yet saved the file. After
saving the page once, what you see on screen matches what's on disk.